This invention relates to inverter apparatus and, more particularly, to inverter apparatus of the simultaneous commutation type.
The inverter apparatus of this type is generally connected as shown in FIG. 1 for supplying three phase alternating current to a load L from a source of direct current connected to input terminals 15 and 16. More particularly, the inverter apparatus I comprises a bridge circuit including Graetz connected controlled semiconductor elements 1 through 6 (since thyristors are generally used, in the following description these elements are called thyristor), and a reverse bias circuit including a pair of power transistors 8 and 9 respectively connected in series with the DC side of the inverter and a pair of diodes 10 and 11 connected across the DC circuit. Diode 10 is connected between positive terminal 15 and the cathode side of thyristors 4, 5 and 6 whereas diode 11 is connected between the negative terminal 16 and the anode side of thyristors 1,2 and 3. Thyristor 7 and reactor 12 which form the subject matter of the invention will be described later. Current detectors 13, 13a', 13b' and 13c' are provided for the input and output sides of the bridge circuit for detecting direct current and alternating current respectively.
Single phase inverters shown in FIGS. 2 and 3 are also known.
Power transistors 8 and 9 are simultaneously turned OFF at the time of commutating current among thyristors thus causing simultaneous commutation.
In the inverter apparatus of the types described above, thyristors 1 through 6 are turned OFF by applying a reverse bias to the thyristors by utilizing the inverse recovery time of diodes 10 and 11. However, whether the commutation is carried out satisfactory or not is influenced by the characteristics of the diodes and thyristors.
More particularly, the term "inverse recovery time" generally means an interval between an instant when the forward current becomes zero and an instant at which a small current can flow in the reverse direction during a process in which a thyristor or diode recovers its reverse current blocking ability subsequent to the termination of the flow of the forward current and the length of such interval generally amounts to about 20 .mu.s. Thus, the reverse current blocking ability is not recovered immediately after termination of the flow of the forward current and certain inverse recovery time t.sub.REC is necessary.
In the prior art inverter apparatus, since the inverse recovery times of the diodes 10 and 11 and of the thyristors 1 and 6 are substantially the same a reverse bias V.sub.B is applied to thyristors 3 and 4 but since the reverse blocking ability of the thyristors 1 and 6 are not yet recovered no reverse bias would be applied thereto thus often failing satisfactory turning OFF.